Recent studies in our laboratory have provided strong evidence that the major outflow from the arm area of the primate motor cortex is derived not from its widely studied set of large pyramidal tract (PT) cells, but instead from a much larger set of small PT cells, and a separate set of small, corticorubral neurons. Although studied little in the past, these small cell systems appear to give rise to 70-80% of the axons that descend from the arm area of motor cortex to the spinal cord to brain stem, motor relay nuclei. Using single-unit recording techniques and a behavioral task that allows us to train monkeys to generate very precise patterns of forearm muscle force, we have also begun to obtain evidence that these small cell populations participate in the control of dynamically different patterns of forelimb muscle activity - and in part by way of separate subsets of the motorneuron pool - than those (phasic) activities that are controlled by large PT cells. We plan in the proposed research to continue these experiments, in order to obtain a statistically valid picture of the motor activities of cells in pyramidal and corticorubrospinal pathways in the normal animal. We plan then to extend these methods and findings in a way that we hope will be of significance for an understanding of the central reorganization processes that underlie a recovery in motor function following CNS trauma or stroke. Trained monkeys will be subjected to unilateral pyramidotomy or rubrospinal tractotomy, and quantitative measures of forelimb motor performance and EMG activity will be used to document: (a) the deficits that are produced in the voluntary control of 'steady' forelimb muscle forces, and in the control of precise, rapidly changing forces; and (b) the time course and extent of recovery of motor function following transection of each pathway. Semi-chronic, single unit recording techniques will be used in the same animals during the post-lesion period, to examine the kinds of changes that occur in the activity of cells in the surviving motor pathway - at both the level of the motor cortex and the red nucleus-which might underlie the observed recovery in motor function.